The reductive activation of the carbon halogen (C-X) bond is a key step in the transformation of organic halides via bond dissociation. Monochloroacetic acid (MCA), a typical recalcitrant aliphatic halide, was previously shown to be reduced by hydrated electrons (e(aq)), requiring 254 nm irradiation. Here, we demonstrate the efficient hydrogenation of MCA to acetic acid (HOAc) with high selectivity using TiO2 electrons accumulated under the bandgap (3.2 eV) excitation in the presence of methanol as a hole scavenger. The reaction occurs via a single electron transfer in concert with C-Cl cleavage (concerted dissociative electron transfer, DET) to give (CH2COOH)-C-center dot, which is then subjected to the subsequent 1e(-)/1H(+) process to give HOAc. The in situ visible and infrared spectroscopic characterizations reveal the critical role of localized Ti-III states in C-CI activation, whereas conduction band or shallow trapped electrons are not active in this process. The photocatalytic reduction rate slowed down with an increase in the intervening carbon atoms (monohalogenated propionic and butyric acids) between TiO2 surface and the C-X bond, which is attributed to the decrease in the electron transfer driving force. (C) 2017 Elsevier B.V. All rights reserved.